Aerosol properties were measured during an airborne campaign experiment that
took place in June 2006 in West Africa within the framework of the African
Monsoon Multidisciplinary Analyses (AMMA). The goal of the present study was
to investigate a dynamical mechanism able to facilitate the sedimentation of
dust particles from the Saharan Air Layer (SAL) into the boundary layer. A
significant change in the dust particle concentration measured along the
meridian between Niamey (Niger) and Cotonou (Benin) was found in the
boundary layer (~700 m), where the dust particle concentration
increased in a zone where local emission is not possible. Moreover, the
boundary layer top observed with the dropsondes launched with the F-F20
shows a strong relationship with the surface cover anomalies, with higher
Boundary Layer (BL) tops over the warmer surfaces, such as croplands, as
opposed to adjacent forest. A mesoscale atmospheric model with a new on-line
dust parameterization, resulting from the Alfaro and Gomes (2001)
parametrisation and AMMA observations, was used to interpret the impact of
vegetation anomalies on dust particle sedimentation. The results of the
simulation are consistent with the observations, with higher dust
concentration over the warm surface cover anomalies.